Efficient coupling of microwave energy into the bulb is crucial to strongly exciting the contents of the bulb, to cause it to incandesce. For this reason, air wave guides have not been successful for this purpose.
In U.S. Pat. No. 6,737,809, in the name of F M Espiau et al., there is described:
A dielectric waveguide integrated plasma lamp with a body consisting essentially of at least one dielectric material having a dielectric constant greater than approximately 2, and having a shape and dimensions such that the body resonates in at least one resonant mode when microwave energy of an appropriate frequency is coupled into the body. A bulb positioned in a cavity within the body contains a gas-fill which when receiving energy from the resonating body forms a light-emitting plasma. (Despite reference to a “bulb”, this specification does not describe a discrete bulb, separable from the lamp body.)